T. V. Rivkin

Large dielectric constant (ε/ε0>6000) Ba0.4Sr0.6TiO3 thin films for high-performance microwave phase shifters

We deposited epitaxial Ba0.4Sr0.6TiO3 (BST) films via laser ablation on MgO and LaAlO3 (LAO) substrates for tunable microwave devices. Postdeposition anneals (∼1100 °C in O2) improved the morphology and overall dielectric properties of films on both substrates, but shifted the temperature of maximum dielectric constant (Tmax) up for BST/LAO and down for BST/MgO. These substrate-dependent Tmax shifts had opposite effects on the room-temperature dielectric properties. Overall, BST films on MgO had the larger maximum dielectric constant (e/e0⩾6000) and tunability (Δe/e⩾65%), but these maxima occurred at 227 K. 30 GHz phase shifters made from similar films had figures of merit (ratio of maximum phase shift to insertion loss) of ∼45°/dB and phase shifts of ∼400° under 500 V (∼13 V/μm) bias, illustrating their utility for many frequency-agile microwave devices.

Nonlinear behavior of thin film SrTiO3 capacitors at microwave frequencies

The voltage-dependent dielectric constant (e) of SrTiO3 (STO) thin films is the basis for developing cryogenic capacitors for tunable microwave applications. In this study, the effect of microwave signal level on nonlinear response at 1.7–1.9 GHz was examined by measuring the level of the third order intermodulation distortion (IMD) signal relative to the input signal level. Small signal dielectric properties such as capacitance, tuning, and loss (tan δ) were also measured at 1 MHz, 3 GHz, and 10 GHz, at temperatures from 4.2 to 300 K. Planar capacitors were comprised of highly (100)-oriented, 1 μm thick STO films deposited via magnetron sputtering onto CeO2-buffered (11_02)-oriented sapphire substrates, with 10 μm gaps between the electrodes. Deviations from the anticipated cubic dependence of the third order IMD product on incident power, for incident power ranges from −10 to 22 dBm, were attributed to conductivity nonlinearity. At incident power levels of 22 dBm and with no dc bias applied to the capac...

Optimization of SrTiO/sub 3/ for applications in tunable resonant circuits

A series of SrTiO/sub 3/ (STO) films have been grown at a wide range of deposition conditions in order to determine the optimal growth parameters to maximize ferroelectric tuning while maintaining the lowest dielectric losses. The deposition pressure of the ambient O/sub 2/ (40 mTorr<P/sub dep/<800 mTorr) and substrate temperature (750/spl deg/C<T/sub sub/<850/spl deg/C) were varied independently while the fluence was held constant at 2.0 J/cm/sup 2/. The deposited films were characterized by X-ray diffraction (XRD) and low frequency dielectric measurements (1 MHz). The out-of-plane STO lattice parameter (a/sub z/(film)) of the deposited films was found by XRD to be a strong function of the deposition pressure: a/sub z/(film)<a(bulk) at low pressures (P/sub dep/<65 mTorr) and a/sub z/(film)>a(bulk) at higher pressures (P(dep)>65 mTorr). The dielectric constant (/spl epsiv//sub r/) and loss tangent (tan/spl delta/) were determined as a function of applied field at room temperature (300 K) and at liquid nitrogen temperature (77 K). The low frequency dielectric properties of the STO films were found to be a weak function of the strain of the ferroelectric film.

Nanopolar reorientation in ferroelectric thin films

The influence of varying oxygen pressure P(O2) during the growth of Ba0.4Sr0.6TiO3 thin films is investigated using dielectric and local optical probes. A transition from in-plane to out-of-plane ferroelectricity is observed with increasing P(O2). Signatures of in-plane and out-of-plane ferroelectricity are identified using dielectric response and time-resolved confocal scanning optical microscopy (TRCSOM). At the crossover pressure between in-plane and out-of-plane polarization (Pc=85 mTorr), TRCSOM measurements reveal a soft, highly dispersive out-of-plane polarization that reorients in plane under modest applied electric fields. At higher deposition pressures, the out-of-plane polarization is hardened and is less dispersive at microwave frequencies, and the dielectric tuning is suppressed. Nanopolar reorientation is believed to be responsible for the marked increase in dielectric tuning at P(O2)=Pc.

Direct write processing for photovoltaic cells

Direct writing of solar cell components is an attractive processing approach. We have fabricated a 6.8% Si solar cell using silver ink based electrodes. Ohmic contact through the antireflection (AR) coating was obtained with pure Ag electrodes at 850/spl deg/C. We also report on highly conductive silver metallizations and initial results on direct-write TCO demonstrating a 100-micron spatial resolution produced by inkjet printing.

Response time and power handling capability of tunable microwave devices using ferroelectric films

Abstract The microwave (MW) performance of tunable stripline resonators with ferroelectric (SrTiO3 or (Ba, Sr)TiO3) planar capacitors under exposure to high voltage video pulses and MW (10 GHz) signals are studied. The resonator switching times were less than 30 nanoseconds, and these values were limited by the measurement apparatus. The dependence of film capacitance and MW loss in ferroelectric films as a function of MW signal level, and their variation during the pulse, were measured. The large signal MW dielectric constant and loss tangent varied by less than 10% with increasing MW voltage amplitude for MW field strengths up to 1 MV/m in SrTiO3 films, and 0.5 MV/m in (Ba, Sr)TiO3 films.

Spray and Inkjet Printing of Hybrid Nanoparticle-Metal-Organic Inks for Ag and Cu Metallizations

Metal-organic and hybrid metal-organic/metal nanoparticle inkswere evaluated for use in the inkjet printing of copper and silver conducting lines. Pure, smooth, dense, highly conductive coatings were produced by spray printing with (hexafluoroacetylacetonato)copper(I)-vinyltrimethylsilane Cu(hfa)·VTMS) and (hexafluoroacetylacetonato)silver(I)(1,5-cyclooctadiene) (Ag(hfa)COD) metal-organic precursors on heated substrates. Good adhesion to the substrates tested, glass, Kapton tape and Si, has been achieved without use of adhesion promoters. The silver metal-organic ink has also beenused to print metal lines and patterns with a commercial inkjet printer. Hybrid inks comprised of metal nanoparticles mixed with the metal-organic complexes above have also been used to deposit Cu and Ag films by spray printing.This approach gives dense, adherent films that are much thicker than those obtained using the metal-organic inks alone. The conductivities of the silvercoatings obtained by both approaches are near that of bulk silver (2 μΩ·cm). The copper coatings had conductivities at least an order ofmagnitude less than bulk.

Tunable SrTiO/sub 3/ varactors using parallel plate and interdigital structures

The dielectric properties of SrTiO/sub 3/ (STO)/YBCO and STO films deposited on LaAlO/sub 3/ substrates were evaluated, for potential varactor applications wherein the capacitance is altered via a dc voltage applied across the STO film. The optimal growth temperature for the STO films in the STO/YBCO structures was 800/spl deg/C; higher temperatures resulted in interfacial degradation and poor film quality, and lower temperatures resulted in films with lower dielectric constants (/spl epsi//sub r/) and tunabilities. Interdigital varactors comprised of STO films deposited directly on LaAlO/sub 3/ displayed higher tunabilities and significantly lower losses.

Frequency conversion in coplanar waveguide based on BaxSr1−xTiO3 film

We report on both a model and experimental verification of the high frequency nonlinearity for coplanar waveguides (CPW) composed of a dielectric substrate coated with nonlinear dielectric Ba0.4Sr0.6TiO3 films. For the CPW with a propagating microwave signal (f0≈30 GHz), modulation of Ba0.4Sr0.6TiO3 film dielectric constant by a high frequency (fp≈20 MHz) pump signal results in the generation of sidebands with frequency (f0±Nfp). Experimental and simulated spectra of CPW output signals are presented as a function of the direct current bias voltage, pump voltage amplitude, CPW length, and CPW nonlinearity.

Effect of SrTiO3 deposition temperature on the dielectric properties of SrTiO3/YBa2Cu3O7-δ/LaAlO3 structures

Abstract We report on the effect of the deposition temperature of SrTiO3 on the dielectric properties of SrTiO3/YBa2Cu3O7-δ/LaAlO3 thin film multilayer structures. In these structures, the YBa2Cu3O7-δ (YBCO) films were deposited at 800°C by laser ablation, followed by the in-situ deposition of the SrTiO3 (STO) layer at one of the following temperatures: 750°C, 650°C, 550°C, 450°C, 350°C, and 250°C. Gold (Au) films were deposited and patterned on top of the STO layer to form planar Au/STO/YBCO capacitor structures. The electrical response was studied by measuring the dielectric constant (er) and loss tangent (tanδ) of the ferroelectric film from 300–40 K, at 1.0 MHz, and at electric fields up to 100 kV/cm. Our results show 750°C to be a deposition temperature which allows for large variations of er with applied field, and with limited enhancement of tanδ. Lower deposition temperatures cause a reduction of the induced change in er and an increase in tanδ with applied field.